Segment Reporting Information by Segment (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Net sales and revenues	$ 4,300.1	$ 3,820.2	$ 12,524.9	$ 13,053.5
Investment income	7.3	7.8	21.8	24.9
Income before income taxes	440.0	345.4	1,210.1	1,270.7
Depreciation and amortization	206.4	176.2	588.7	522.4
Truck, Parts and Other
Segment Reporting Information [Line Items]
Net sales and revenues	4,006.6	3,546.7	11,649.5	12,252.5
Income before income taxes	344.5	257.2	938.5	1,016.7
Depreciation and amortization	92.9	79.1	262.2	245.6
Truck, Parts and Other | Trucks
Segment Reporting Information [Line Items]
Revenue	3,416.6	3,016.3	9,924.6	10,710.1
Income before income taxes	242.5	170.2	645.9	742.6
Depreciation and amortization	89.2	75.1	250.9	233.4
Truck, Parts and Other | Trucks | Parts and Trucks
Segment Reporting Information [Line Items]
Revenue	(155.4)	(156.4)	(460.1)	(572.1)
Truck, Parts and Other | Trucks | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	3,261.2	2,859.9	9,464.5	10,138.0
Truck, Parts and Other | Parts
Segment Reporting Information [Line Items]
Revenue	726.2	661.4	2,126.0	2,031.1
Income before income taxes	106.5	88.7	311.2	280.2
Depreciation and amortization	1.2	1.4	3.9	4.5
Truck, Parts and Other | Parts | Parts and Trucks
Segment Reporting Information [Line Items]
Revenue	(10.9)	(11.3)	(33.8)	(34.4)
Truck, Parts and Other | Parts | Revenues From External Customers
Segment Reporting Information [Line Items]
Net sales and revenues	715.3	650.1	2,092.2	1,996.7
Truck, Parts and Other | Other products
Segment Reporting Information [Line Items]
Net sales and revenues	30.1	36.7	92.8	117.8
Income before income taxes	(4.5)	(1.7)	(18.6)	(6.1)
Depreciation and amortization	2.5	2.6	7.4	7.7
Financial Services
Segment Reporting Information [Line Items]
Net sales and revenues	293.5	273.5	875.4	801.0
Income before income taxes	88.2	80.4	249.8	229.1
Depreciation and amortization	$ 113.5	$ 97.1	$ 326.5	$ 276.8